Musical instrument tuning apparatus



1960 H. A. POEHLER USICAL INSTRUMENT TUNING APPARATUS 3 Sheets-Sheet 1Filed March '7, 1955 m s. 3mm x .u 1% /////r 1960 H. A. POEHLER2,958,250

MUSICAL INSTRUMENT TUNING APPARATUS Nov. 1, 1960 H. A. POEHLER 2,958,250

MUSICAL INSTRUMENT TUNING APPARATUS Filed March 7, 1955 3 Sheets-Sheet 3INVENTOR. Horsf' 14/6/17 Posh/er United States Patent MUSICAL INSTRUMENTTUNING APPARATUS Horst Albin Poehler, 12 Manville Lane, Pleasantville,N.Y.

Filed Mar. '7, 1955, Ser. No. 492,467

9 Claims. (Cl. 84-454) This invention relates to an improved device anda method for tuning musical instruments, and, more specifically, to adevice which visually indicates the agreement of an audible note of amusical instrument with any audible standard of pitch.

This invention is applicable to the tuning or intonation of allpresently-used musical instruments, as well as to musical-instrumentresearch and design. In general, the invention is applicable to musicalinstruments emitting any audible note, whether the note be nearly pure,contains many partials, or contains other tones in addition to partials.In addition to operation on the fundamental of the tone, the device maybe employed for the inestigation of any of the harmonies or partials ofthe one.

An object of this invention is to provide a simplified, visualindication of musical pitch.

Another object of this invention is to provide an improved method andapparatus for tuning pianos and, in particular, for stretching theoctaves.

Another object of this invention is to provide an apparatus that willgive a visible indication of the presence, relative magnitude, anddeviation from a true harmonic series, of the overtones of musicaltones.

Another object of this invention is to provide a visual means fortesting the intonation of instrumental and vocal tones.

Another object of this invention is to provide music teachers with animproved apparatus for teaching and illustrating musical intervals andthe perception of beats.

Another object of this invention is to provide music teachers with aninstrument that gives a simplified visual indication of vibrato.

While the invention is generally useful in the field of music, oneapplication is the tuning of pianos. Tuning procedure, as generallypracticed, requires a great skill, not possessed by all piano tuners. Itinvolves tuning the temperament, or middle octave, by tuning one note toa standard pitch. The other notes of the temperament octave are thentuned relative to the first note, and relative to each other, by ear. Indoing this the tuner must hear faint beats between second, third, orfourth harmonies of two notes of the temperament when struck together.This involves, for example, tuning D to 293.66 vibrations per second.When A.,,, having a fundamental frequency of 440.00 vibrations persecond, is sounded simultaneously with D, at 293.66, the second harmonicof A 880.00 and the third harmonic of D 880.98, produce a beat whosefrequency is 0.98 beats per second or 59 beats per minute. The tunermust judge the frequency of these beats. He must adjust the frequency ofD until the correct beat frequency is heard. Thus any error of judgmentof the frequency of this hardly-discernible low beat between weakharmonic frequencies, in the presence of the strong fundamentals, willproduce an error in the tones being tuned. Not only must the piano tuneroperate as a human filter to fix his attention on a single faint beatnote to the exclusion of a number of much stronger masking tones, but hemust also, and at the same time, be a precision timer in order todifferentiate 59 cycles per minute from 57 or 61.

The piano tuner encounters still another difficulty in tuning pianoswhich are in poor condition. Sometimes, due to rusted or twistedstrings, or some peculiarity of one or more of the hammers, a defect inthe sounding board, or due to some other cause, a piano note is ragged,having a peculiar tonal quality, which may make it nearly or quiteimpossible to pick out a desired higher harmonic by ear.

This invention avoids the difliculties inherent in detecting the beatsbetween the harmonics of notes in the temperament. It provides twelvefrequencies that are spaced in the same ratio as the chromatic musicalscale, but that can be moved up and down in frequency as a group. Withthis device, tuning the temperament of a piano may be reduced simply tosetting the notes of the temperament to coincide with the twelvefrequencies supplied by the apparatus in accordance with this invention.Coincidence of the piano string vibration with the correspondinggenerated frequency is rendered by an improved, simple visualpresentation. It has long been established that the second partial (orfirst overtone) of a piano string does not occur at twice the frequencyof the fundamental, but has a somewhat higher pitch. This inharmonicityof the second partial of a vibrating piano string varies from piano topiano, and from string to string on a given piano, being more pronouncedin the upper and lower registers of the piano keyboard, than in themiddle. In the past, piano tuners have taken the inharmonicity intoaccount, to a degree, by the common practice of tuning the upperregister somewhat sharp and the lower register somewhat fiat. Thisprocedure is commonly referred to as stretching the octave. However, thepiano tuner, again, is faced with the difficult problem of detectingfaint beats in the presence of stronger tones, which tend to obscure ormask weak beats. With this invention, stretching of the octaves can beaccomplished in a rational manner, by enabling the tuner to tune a givennote to the same frequency as the second partial of the note one octavebelow, or to the frequency of the fourth partial of the note two octavesbelow.

The objects and advantages of the invention discussed above, and others,will become more apparent from the following description andaccompanying drawings, forming part of the application.

In the drawings:

Fig. 1 is a schematic circuit of one embodiment of the apparatus inaccordance with the invention,

Fig. 2 is a schematic circuit of one form of oscillator that may be usedas part of the apparatus of Fig. 1,

Fig. 3 schematically depicts an audio amplifier which may be used withthe apparatus of Fig. 1,

Figs. 4, 5, and 6 represent various cathode ray tube screen patternsemployed in tuning musical instruments by the use of the apparatus ofthe invention,

Fig. 7 is a front view of the control panel of the apparatus, inaccordance with the invention.

Broadly, the invention, in one of its forms, comprises: a cathode raytube whose deflected beam is swept circularly at any one of twelvediscrete frequencies, whose spacin is exactly equal "to the spacing ofthe twelve tones of the chromatic, equally-tempered, musical scale, butwhich frequencies may be moved up and down as a group, without alteringsaid spacing, an oscillator to supply these frequencies, one audioamplifier to energize one pair of the cathode ray tubes deflectionplates, a phase shifting network, a second audio amplifier to energizethe other pair of the cathode ray tubes deflection plates, atpreamplifier to amplify the weak electrical signals produced by themicrophone and to lower the impedance level, a tunable bandpass filter,and, a delayed automatic gain control amplifier to control the intensityof the deflected beam.

In this form of the invention, a microphone is employed to pick up,through the air, the vibrations emitted by a selected musical tone. Theelectrical signal produced by the microphone is amplified in thepreampliher and passed, at a low impedance level, to an adjustableband-pass filter which rejects ali frequencies except the fundamental,or any particular desired partial, of the musical tone. The electricalsignal passed by the filter, is further amplified in the delayedautomatic gain amplifier and applied to a control grid of the cathoderay tube, thereby modulating the intensity of the deflected beam. Thedelayed automatic gain control feature is useful, since it stretches thetime interval during which tones that fade out rapidly are displayed forstudy. The biases of the cathode ray tube are so adjusted that in theabsence of the control grid signal, the intensity of the deflected beamis such as to make the pattern on the screen of the cathode ray tubejust barely visible. Positive portions of each cycle of the amplifiedtone signal brighten corresponding parts of the pattern; negativeportions darken corresponding parts of the pattern. The circular patternis produced by the action of the oscillator, the phase shift network,and

the two audio amplifiers. The output of the oscillator is split into twochannels. In the one channel, the oscil'lator output is amplified andapplied to one pair of the cathode ray tubes deflection plates. In theother channel, the oscillator output is shifted approximately 90 inphase by the phase-shift network, amplified by the second audioamplifier and applied to the other pair of deflection plates, which areat right angles (90) to the former pair. In a manner well known to theart, this application of signals, equal in amplitude, and differing inphase by 90 results in a circular pattern. If, then, the frequency ofthe musical tone be the same as the oscillator frequency, a stationarysection of a circle will appear on the screen. If, however, the twofrequencies are slightly different, the illuminated section of thecircle will rotate in a direction indicating which frequency is thehigher, and at a speed in revolutions per second corresponding to thefrequency difference. If the frequency applied to the control grid beexactly double that of the oscillator, two opposite sections of thecircle will be illuminated and stationary; and, in general, if thecontrol grid frequency is n times the osciliator frequency, where n isan integer, the circle will have n illuminated sections at equalintervals. sections will app ar to rotate if the frequencies beingcompared do not have an integral relation. For control grid frequencyl/n times the oscillator frequency, the circle will have one illuminatedsection.

In tuning a piano note to the second partial of a note an octave lower,the filter selects and passes the second partial of the lower note andattenuates the fundamental. This has the efiect of preventing thepattern of the fundamental of the tone from appearing on the cathode raytube face to mask the fainter, second partial representation.

The apparatus of the invention is, not only, useful for tuning pianosand other musical instruments, but also constitutes a reliable andeasily-used device for making adjustments in pitch and checking theintonation of any musical instrument.

Testing the intonation of a musical instrument is readily accomplishedwith the apparatus of this invention. The apparatus has twelve keys orpush buttons, which correspond to the notes of the chromatic musicalscale. The player sounds a note on his instrument and depresses thecorresponding button of the apparatus, thereby circularly sweeping thescreen of the cathode ray These tube at the frequency corresponding tothe button that is depressed. The frequency at which the circle is sweptout is then adjusted to beexactly equal to the frequency of the musicaltone that is being sounded and picked up by the microphone by tuning thefine frequency control until a stationary pattern appears on the face ofthe cathode ray tube. The player, then, plays each note of theinstrument in sequence, depressing for each note the correspondingbutton of the apparatus, and avoiding any further adjustment of thefrequency of the apparatus. If the intonation of the instrument iscorrect, a stationary pattern will be obtained for all the notes, and ifnot correct, the apparatus will indicate by the direction of therotation of the visual pattern, whether the intonation of a particularnote is flat or sharp.

A method of teaching tone perception with the apparatus of thisinvention is to sound a note on some musical instrument. Thecorresponding button on the apparatus is depressed, thereby circularlysweeping the face of the cathode ray tube at a frequency correspondingto the button that is depressed. The frequency at which the circle isswept out is then adjusted to be exactly equal to the frequency of themusical tone that is being sounded and picked up by the microphone byturning the fine frequency control until a stationary pattern appears onthe face of the cathode ray tube. If the singer or instrument player,thereafter, sounds the required pitch exactly, a stationary pattern willbe obtained, but a flat tone would be evidenced, for example, by acounter-clockwise rotation, and a sharp tone by a clockwise rotation.

In addition, the invention has several applications in connection withthe teaching of music. For this purpose an audio output is useful. Forexample, in voice training the people may be required to sound and holda tone. The same oscillation that sweeps out the circular trace on thecathode ray tube is fed through a volume control to an audio amplifierand, by means of a loud speaker, produces an audible tone. The tone isthen turned off and the singer is required to sound the same tone, or atone an octave higher or lower. A switch is provided to blank thecathode ray trace until the singer has established the tone. Otherwise,the watching singer would unconsciously vary her pitch until the patternstands still. Releasing the switch brings the pattern into view;

and, if the singer is on pitch, a stationary, single-seg ment patternwill be obtained. A stationary, two-segment pattern will be obtained ifthe singer correctly sings an octave higher. A fiat tone would beapparent by a counterclockwise rotation, for example, and a sharp toneby a clockwise rotation.

In the use of wind instruments the position and tension of the lipsoften modifies the pitch. In the case of the flute a slight roll of theinstrument against the lips changes the pitch. Students must have cartraining to become conscious of the slight changes in pitch so caused,and to learn when the pitch is correct. The situation is complicated byadditional pitch change caused by temperature as the wind instrument isused. The device of twelve tones of the chromatic musical scale, butthat can be moved up and down in frequency, as a group. Means areprovided for the adjustment of the pitch so that the frequencies from E(246.9 c.p.s.) to C (523.3 c.p.s.) are covered. However, this adjustmentis applied to all j twelve so that, for each setting of the adjustment,the separation between the twelve distinct frequencies will alwayscorrespond to the separation of the notes of the.

musical scale. The oscillator output is passed from conductor 12 throughcoupling capacitor 13 into two separate electrical channels, in one ofwhich the output is phase shifted approximately 90 by a simple RCnetwork, resistor 14 and capacitor 16 being provided for this purpose.The output at junction 17 is connected to a voltage divider 18 andthrough one amplifying triode 19 to one set of deflecting plates 22 of acathode ray tube 23. The other oscillator output channel consists of avoltage divider 24 and atriode amplifier 26 connected to the other setof deflecting plates 27 of cathode ray tube 23. The two voltage dividersare required to equalize signal amplitudes fed to the cathode ray tube.This form of two-phase circuit produces a circular trace on the cathoderay tube screen 28.

A switch 35, when closed, biases the control grid 36 to cut ofi, thusblanking the screen of the cathode ray tube.

The cathode ray tube 23 is of the electrostatic deflection type but theelectromagnetic type may be substituted with appropriate changes in thedeflection circuits.

The bias circuits include a negative 700 volt source represented byterminal 29 and applied to a grounded voltage-dividing resistor 30. Oneslider 31 of the voltage dividin resistor 39 is connected to the focusgrid 33, another slider 32 is connected through grid leak resistor 34 tothe first or control grid 36 for brightness adjustment, and grids 37 and38 are grounded. Potentials are so arranged that in the absence of acontrol grid signal the screen is just slightly illuminated.

A microphone 39 is arranged to pick up the tone of a piano string, orother musical instrument which is to be tested. The microphone 39 iscoupled through capacitor 41 to the grid 42 of the pentode amplifier 43,with output coupled from its anode 44 through a capacitor 46 to a grid47 of a triode cathode follower 48. The purpose of the cathode followeris to lower the signal source impedance so that the signal may be fedinto a series LC filter, without degrading the high Q of the LC filtercircuit. The cathode 49 of the cathode follower is connected to a sevenpoint switch 51, which permits any one of seven capacitors 52, 53, 54,56, 57, 58, 59 to be selected. Since these capacitors correspond to theseven octaves of the piano, they are labeled octave 1, 2, 3, 4, 5, 6, 7.The series resonant circuit is connected in series with a resistor 61,whose resistance is low compared to the resistance of the tappedinductor 62. One end of resistor 61 is grounded and the other end isconnected in series with the inductor and is connected to the contact 63of switch 65. The voltage across the resistor 61 is proportional to thecurrent through it. The current is maximum at the frequency for which Land C are resonant and falls off rapidly for frequencies higher or lowerthan the resonant frequency, provided the Q of the series LC is high.This resonant frequency can be chosen to coincide with any of eightyfour notes of the piano, since the switch 6%: permits any of the twelvenotes of the chromatic musical scale to be selected, while switch 51,independently, permits any of the seven octaves of the piano keyboard tobe selected. The twelve taps of the inductor 62 are so chosen that theresonant frequency of the series LC filter will coincide with thefrequencies of the chromatic musical scale from C (261.6 c.p.s.) to B;(493.9 cps.) for the LC combination made up of the center capacitor C 56and the twelve inductances of the twelve taps of inductor 62. The twelvetaps of the inductor are labeled, therefore, as C, Cit, D, Dll, E, F,Fl, G, Gti, A, At, B. The filter need only cover seventy six of theeighty eight notes of the keyboard, since only the partials, and not thefundamentals, of the lowest octave are used in tuning the notes of thelowest octave. The filter of Fig. 1 provides eighty four positions,hence eighty four frequencies may be selected. In the special case whereeighty eight filter positions are re- 6 quired, it will suffice to addanother capacitor and another contact to switch 51.

The output of the filter is applied to contact 63 of a single-pole,double-throw switch 65. The switch permits the signal to be useddirectly without being passed through the filter, since the cathode 49is connected to terminal 64 of switch 65.

The output of switch 65 is coupled by capacitor 66 to the grid 67 of aremote-cutoff pentode 68. The output of pentode 68 is coupled bycapacitor 69 to the grid 71 of another remote-cutoff pentode 72. Thebias of both pentodes 68 and 72 is controlled by a negative, DC. voltagedeveloped by diode 73, from the signal amplified by the pentode 74.

The output of pentode 72 is coupled by means of capacitor 77 to the grid36 of cathode ray tube 23.

Delayed automatic gain control of the amplified musical tone picked upby the microphone is desirable for most applications. Tones that die outrapidly, such as the tone of a struck piano string, for example, wouldotherwise not give an indication of sufiiciently long duration to beuseful for tuning. In effect, automatic gain control stretches the timethat a musical tone can be studied. Initially, when the sound is loud,the amplifier has a low gain, due to the large negative bias developedby diode 73 and applied to the grids of pentodes 68 and 72. As the tonedies out, the bias developed by diode 73 decreases, and hence the gainof the amplifier increases. in practice, therefore, a useful signal isobtained at the grid 36 of the cathode ray tube 23, as long as thesignal of the musical tone is above the noise level.

The automatic gain control consists of a separate pentode amplifier 74and a diode rectifier 73. The same signal that is applied to the grid 67of the remote-cutoff pentode 68 is applied by means of capacitor 78 tothe grid 79 of pentode amplifier 74. The amplified output from this tubeis taken from its plate 81 and coupled to one end of a resistor 84 bymeans of capacitor 82. The automatic gain of the microphone amplifiershould be delayed, since we do not wish to reduce gain, as the signalincreases, for small input signals. Delayed automatic gain control isachieved by biasing the cathode 83 of diode 73 with a suitable positivevoltage. Diode 73 conducts whenever the A.C. signal voltage developed atterminal 84 is sulficiently positive to overcome the positive biasapplied to cathode 83. Current pulses which flow for positive-goingsignals cause terminal 84 to take on a pulsating negative potential.Resistor 86, together with capacitor 87, serves to filter out A.C.pulsations from the bias voltage developed by the rectification of diode73. The bias is applied to bus which leads to the. grid resistors ofremote-cutolf pentodes 68 and 72. Further filtering is supplied byresistor 90 together with capacitor 91. The bias supplied by bus 90' isnegative and increases in amplitude as the signal at grid 67 increases.Hence, as the signal at grid 67 increases, the bias on bus 90 alsoincreases, becoming more negative. The increased negative bias producesa decreased transconductance in pentodes 68 and 72. Hence the signaloutput at the plate 76 remains relatively constant for a wide range ofinput signal amplitudes at grid 67. In this manner, automatic gaincontrol is achieved.

One form of oscillator 11 which satisfies the requirements of theinvention is the negative feedback, resistanceoapacitor oscillator shownin Fig. 2. It is significant to note that the apparatus, in accordancewith the invention, does not require an oscillator that has absolutefrequency stability. It requires merely an oscillator of good frequencystability. Absolute calibration is achieved by periodic checking againsta suitable. standard such as WWV or a precision tuning fonk. It isimportant that the oscillator maintain the relative ratios of the twelvefrequencies to a high degree of precision, the ratio of the frequenciesbeing in the ratio of the chromatic musical scale. Further, it isdesirable to be able to move the 12 7 frequencies up and down infrequency as a group, without disturbing their ratios to each other.

The above requirements are met by the particular form of theresistance-capacitance oscillator shown in Fig. 2. The requirements mayalso be met by other oscillators, such as a pentodenegative-transconductance oscillator using a toroidal inductor withtaps, that will provide frequencies in the ratio of the chromaticmusical scale.

Frequency stability is achieved by the use of low temperaturecoefficient, wire-wound resistors for the frequency determining RCnetwork. For the capacitor of this RC network, high-Q silver-micacondensers of low temperature coefficient are used. Since it is possibleto obtain higher stability with resistors than with capacitors, a gangof resistors is used as the frequency selecting element. By usingresistors of the same kind in the resistor gang, whatever smallpercentage change takes place in the value of one resistor will takeplace in all resistors, and hence even though the absolute value offrequency may vary slightly, the ratios of the twelve frequencies toeach other are maintained to a high degree of precision.

In Fig. 2, two banks of resistors are connected to twelve interlockingkey switches, one switch for each tone of the chromatic musical scale.End resistors 171 and 172 of the two gangs are connected to twoseparated, normally-open contacts 173 and 174 of a switch having a key176. When key 176 is depressed, it locks down, connecting both contacts173 and 174 to the common bus bar 177. The depression of key 176 alsoreleases any other previously-depressed key. These twelve interlockingkeys are designated C, Ci, D, Di, E, F, Fit, G, Git, A, At, and B.Oscillator 11 produces twelve frequencies which are merely nominallyequal to, but which do have precisely the same spacing in the frequencydomain as have the frequencies of the tones of the middle octave ortemperament of the piano, starting with middle C or C which has afrequency of 261.6 cps, and ranging to the first B above middle C, whichis B (493.9 c.p.s.). These twelve frequencies may be moved up and downas a group without altering their spacing.

Resistors of the first gang, including resistor 171 have one terminalgrounded, the other terminal being connected through the key that isdepressed through conductor 177 to the control grid 178 of the pentodeoscillator tube 179. Any one of these resistors, as connected into thecircuit, is connected in shunt with capacitor 80. T he components thusconstitute a shunt resistance-capacitance circuit between the grid 178and ground, having a definite time constant. Adjustable capacitor 85 isused in tuning the oscillator frequency and effects the same percentagechange in all of the twelve frequencies of the oscillator 11. Hence theknob 85 control-ling condenser 85 may be calibrated in cents (one centequals semitone). This is highly desirable, since it makes it possibleto compensate for a sharp or fiat tone by turning the knob 85', andthereafter to read the degree of sharpness or flatness in cents directlyon the dial. Hence, adjustable capacitor 85 serves to move the twelvefrequencies up and down as a grow without altering their spacing.

Resistors of the second bank including resistor 172 are connectedthrough one terminal and the associated key, when depressed, to grid bus177 and to control grid 178. The other resistor terminals are joined tobus 36 which is connected through a fixed capacitor 88 to the oscillatoroutput terminal 89. The selected resistor of this bank, thereforeconstitutes with capacitor 88 a series resistancecapacitancecombination. A tungsten filament lamp 95' is connected between thecathode 92; of pentode 17? and ground, and serves as a cathode resistorfor automatic control of oscillator output amplitude. The output ofoscillator tube 179 is taken from its anode 93 and is amplified inpentode amplifier and phase inverter 94 before being coupled throughcapacitor 96 to the output terminal 89. Regenerative coupling isprovided, and. the

frequency is determined by the resistancecapacitance network composed ofcapacitors 88, and and the resistor gangs of which resistors 171 and 172are a part. Oscillation takes place at the frequency for which thevoltage fed back is in phase with the input voltage. Adjustable resistor97, in series with fixed resistor 98 to the cathode 92, is provided toallow compensation for differences in the tungsten lamp element 95.

In order to train pupils in the perception of pitch, and for otherpurposes, an oscillator audio output is useful. The simple circuit ofFigure 3 is suitable. Conductor 107, is connected from the outputterminal 89 of the oscillator to a volume-control potentiometer 109. Anamplifying tube 111 has its control grid 112 connected to the slider 113of the volume control operated by knob 109', and has an outputtransformer 1-14 in its plate circuit, operating a loud speaker 116. Aswitch 117 allows the audio output to be turned completely off.

Figure 7 shows a front view of the control panel of the apparatus, inaccordance with the invention. The twelve black and white keyscorrespond to the notes of the chromatic musical scale, and are coloredblack and white in accordance with the piano keyboard, the accidentalsbeing black. The panel is furnished with the following controls: pitchcontrol knob 85, calibrated in cents, two switches of the filter whichconsist of an octave-selector switch 51 and a note-selector switch 60operated by knobs 51' and 60 respectively, a brightness control knob32', a volume control knob 169, which is provided with an on-off switch117, and a filter switch 65. In addition to the above controls, thefront panel contains the face 28 of the cathode ray tube 23.

In operating the apparatus of the invention, the oscillator, Fig. 2, isfirst calibrated with some suitable standard of frequency. As anexample, we shall use a standard tuning fork, and shall calibrate theoscillator to middle C (C in the following manner. With switch 65,Figure 1, in the 63 position, the filter is tuned to the fundamental ofC by setting the switch 51 to the capacitor 56 position, and setting theswitch 60 to the C tap. As a result the control grid 36 of cathode raytube 23 will be actuated by the fundamental of the tone impinging on themicrophone 39. The oscillator key C, Fig. 2, is depressed. The C tuningfork is struck, and the oscillator is adjusted by adjusting capacitor 85until a stationary single segment 134, Figure 4, of a circle is seen onthe screen 28, Fig. 1. The oscillator has now been calibrated, and willrequire no further adjustment unless the setting of capacitor 85 ischanged. The dashed part 133 of the circle in Fig. 4, represents thedarkened portions of the pattern. The brightened section, 134, Fig. 4,represents the portion that is intensified during that part of eachcycle when the control grid 36, Fig. 1, is of positive polarity. Acounterclockwise rotation of segment 134, Fig. 4, for example, indicatesthat the tuning fork frequency is lower than that of the oscillator, anda clockwise rotation indicates that the frequency is higher.

The next step, and for a piano tuner the most important and difficultstep, is the tuning of the middle octave, or temperament, of the piano.The note C, will be tuned first. The C key of the piano is struck, whichwill again cause part of the circle pattern, Fig. 4, to be illuminatedand, in general, to rotate rapidly. The three C strings are now tuned,one after the other. In each case, the other two strings of the note aremuted with a rubber Wedge, and the tension on the string being tuned isadjusted until the rotation of segment 134, Figure 4, is stopped.

To tune the next note, C11 the oscillator key Cit, Fig. 2, is depressed.The calibration of the oscillator with a tuning fork is not required atthis point, since the oscillator has the property that its twelvefrequencies have a spacing exactly equal to the spacing of the twelvetones of the chromatic, equally-tempered, musical scale.

9 Hence, calibration of any one of the twelve frequencies of theoscillator will suffice. The filter is set to the Cit, position bysetting the switch 51 to the capacitor 56 position and setting theswitch 60 to the Cii tap. The Cit; key of the piano is struck, whichwill again cause the 134 segment, of the circle pattern Figure 4 toappear and to rotate. The three strings of C31 are then tuned, one at atime, until in each case the rotation of the segment 134 is stopped.

The operation of tuning the three strings, is then repeated in turn,with each of the ten remaining notes of the middle octave of the piano.

The two-position switch 65 may be kept connected to terminal 64 intuning each note of the middle octave, thus omitting the filter.Although the filter is needed in the tuning of the higher and loweroctaves, it may be dispensed with in the tuning of the middle octave.Its use for the middle octave, however, produces a clearer screenpicture, because the numerous overtones that are generated when a stringis struck would cause the darkened part 134, Figure 4, to becomepartially illuminated.

To tune C an octave higher, to C employing the oscillator of Figure 2,set the filter of Figure l to C by setting switch 51 to the capacitor 57position, and switch 60 to the C tap, and set the oscillator, Figure 2,to C by depressing the C key, 175. The piano key C is now struck. Themicrophone, filter, and amplifier will now apply the second partial, andnot the fundamental, of C to the control grid 36 of cathode ray tube 23,Figure 1, making a second-partial pattern, composed of two oppositebright sections of circle 136 and 137, as indicated in Figure 5. Thesesegments will, in general, rotate. In filtering out the second partial,we have set the filter to the C position. As we have pointed out,however, the second partial of C is somewhat higher in pitch than CActually the second partial of C is sufficiently close to C that thefilter will pass the second partial of 0, when set to the C position,while at the same time sufficiently attenuating the other partials. Asimilar explanation applies to the other filter positions. Theoscillator is then adjusted to stop the rotation of the two segments, byreadjusting the capacitor 84, Figure 2. Piano key C is now struck. Thetwo-segment pattern that is seen will, in general, be rotating, and thethree strings of C are tuned, one at a time, with the other twomuted,until the pattern stops rotating. This procedure is repeated with theother notes of the first octave above, and is repeated in an analogousmanner with all the notes of the higher octaves. In each case the finefrequency capacitor 85 is readjusted so as to make the patternstationary for the second partial of the note an octave below, andthereafter this adjustment of capacitor 84 is maintained while adjustingthe fundamental of the upper note.

In tuning notes of the second octave above the middle octave, thepattern has four segments, as shown in Figure 6, and in tuning the notesof the third octave above, the pattern has eight segments.

In tuning the bass notes of the piano relative to the notes of thetemperament, two methods are commonly employed by tuners. One method isto tune by octaves, and the other method is to tune by double octaves.In the former method, the second partial of the lower note should bemade to Zero beat with the fundamental of the upper note. In the lattermethod, the fourth partial of the lower note should he made to zero beatwith the fundamental of the upper note. The apparatus and methoddescribed in this invention are applicable to either of the abovemethods of tuning, or, indeed, to any other methods where partials oftones are involved in tuning. Since the above two methods of tuning areidentical except for the use of the fourth instead of the second partialin the tuning, only the first of the two methods, that of tuning to thesecond partials will be described in detail.

v The tuning of the bass notes of the piano to the notes of the middleoctave, or temperament, by octaves, will be illustrated in the followingexample. To tune C one octave below C to C for example, the oscillatoris set to C by depressing the C key 175, Figure 2. The filter is set toC by setting the switch 51 to the capacitor 56 position, and the switch60 to the C tap. The C piano key is now struck. Since the filter is setto exclude the fundamental of C and any other partials, only the secondpartial will pass the filter. This will produce a single-segment patternon the screen. The pattern will in general, rotate. TheC strings are nowtuned one at a time until the rotation of the pattern ceases. Thisprocedure is followed with all the other notes one octave below themiddle octave of the piano.

To tune the notes of the second octave below the middle octave, forexample note C the filter is set to C by setting the switch 51 to thecapacitor 54 position, and the switch 60 to the C tap. The oscillator isset to C by depressing the C key 175, Figure 2. A singlesegment patternwill appear when C is struck corresponding to the fundamental of C Thecathode ray tube control grid brightens the screen on alternate sweepsonly. The oscillator is adjusted until the pattern is stationary, byadjusting the capacitor 85, Figure 2. The piano note C is now struck. Asingle segment pattern, representing its second partial, will beobserved. The C strings are then tuned, one at a time, until the patternceases its rotation. The remaining bass notes are then tuned in ananalogous manner.

The invention, as described above, provides a highlystable, dependable,and relatively inexpensive device that greatly facilitates the tuning ofmusical instruments, particularly pianos. With the improved oscillator,which has push button control for coordination with the piano keyboard,twelve frequencies are produced that will precisely maintain theirspacing, in the frequency domain, which is equal to the spacing of thetwelve tones of the chromatic musical scale, notwithstanding temperaturedifferences, and within an accuracy greater than that which can beattained by any listening process as presently employed in the tuning ofinstruments. Moreover, a single frequency standard, such as a tuningfork, is all that is needed in order to properly adjust all twelvefrequencies to an absolute standard. After this adjustment all the notesof the piano can be tuned without the aid of any additional standards,as previously described. Another important feature of the inventionresides in the provision of the visual presentation of the correlationof the frequencies so that the operator can immediately ascertainwhether or not the two frequencies are of the same pitch, and if notwhich one is flat.

The term a simple closed curve, where used herein shall be understood tohave the following definition: a simple closed curve is a curve thatdoes not intersect itself, and, therefore, is one that divides the planeinto exactly two domains, an inside and an outside.

While only one embodiment, of the invention has been illustrated anddescribed, it is apparent that many modifications, alterations andchanges may be made without departing from the true scope and spiritthereof.

What is claimed is:

1. In musical instrument tuning apparatus, the combination of anoscillator for producing a plurality of discrete oscillations Whosespacing in the frequency domain is equal to the spacing of the tones ofthe chromatic musical scale, means for adjusting said oscillator toshift said frequencies over a range as a group to reproduce thefundamental tones of a musical scale of at least two octaves, a cathoderay tube having means to generate an electron beam and including acontrol grid for controlling the intensity of said beam, deflectingmeans for deflecting said beam, and a screen responsive to said beam,means for applying a selected frequency of said oscillator to saiddeflecting means for moving the said beam to sweep out an ellipse onsaid screen at the periodicity of said selected frequency, a pick-updevice for translating the sound wave produced by a musical instrumentbeing tuned into electrical signals, means for amplifying saidelectrical signals, and means for applying the amplified electricalsignals to said control grid of said cathode ray tube to intensitymodulate the said electron beam as it moves to sweep out the saidellipse, whereby a non-rotating, visible pattern is produced on saidscreen when an integral relation exists between the frequency of thesaid sound wave and said selected frequency sweeping out the saidellipse.

2. In musical instrument tuning apparatus, the combination of anoscillator for producing a plurality of discrete oscillations whosespacing in the frequency domain is equal to the spacing of the tones ofthe chromatic musical scale, means for adjusting said oscillator toshift said frequencies in predetermined steps over a range as a group toreproduce the groups of tones of at least two octaves of the musicalscale, a cathode ray tube having means to generate an electron beam, andincluding a control grid for controlling the intensity of said beam,deflecting means for deflecting said beam, and a screen responsive tosaid beam, means for applying a selected frequency of said oscillator tosaid deflecting means for moving the said beam to sweep out an ellipseon said screen at the periodicity of said selected frequency, a pick-updevice for translating the sound waves produced by a musical instrumentbeing tuned into electrical signals, amplifying means including a filterfor amplifying and filtering the said electrical signals, and means forapplying the amplified and filtered electrical signals to said controlgrid of said cathode ray tube to intensity modulate the said electronbeam as it moves to sweep out the said ellipse, whereby a non-rotating,visible pattern is produced on said screen when an integral relationexists between the frequency of the filtered components of the saidsound waves and said selected frequency sweeping out the said ellipse.

3. In musical instrument tuning apparatus, the combination of anoscillator for producing a plurality of discrete oscillations whosespacing in the frequency domain is equal to the spacing of the tones ofthe chromatic musical scale, means for adjusting said oscillator toshift said frequencies over a range as a group to reproduce thefundamental tones of a musical scale of at least two octaves, a cathoderay tube having means to generate an electron beam, and including acontrol grid for controlling the intensity of said beam, deflectingmeans for deflecting said beam, and a screen responsive to said beam,means for applying a selected frequency of said oscillator to saiddeflecting means for moving the said beam to sweep out an ellipse onsaid screen at the periodicity of said selected frequency, a pick-updevice for translating the sound wave produced by a musical instrumentbeing tuned into electrical signals, amplifying means including anadjustable bandpass filter for amplifying the said electrical signalsand allowing only those components of the said electrical signal thatlie within the pass region of the said adjustable bandpass filter topass, and means for applying the amplified and filtered electricalsignals to said control grid of said cathode ray tube to intensitymodulate the said electron beam as it moves to sweep out the saidellipse, whereby a non-rotating, visible pattern is produced on saidscreen when an integral relation exists between the filtered frequencycomponents of the said sound wave and said selected frequency sweepingout the said ellipse.

4. In musical instrument tuning apparatus, the combination of anoscillator for producing a plurality of discrete oscillations whosespacing in the frequency domain is equal to the spacing of the tones ofthe chromatic musical scale, means for adjusting said oscillator toshift said frequencies over a range as a group to reproduce thefundamental t nes of musical scale of at least two octaves, acathode raytube having means to generate an electron beam, and including a controlgrid for controlling the intensity of said beam, deflecting means fordeflecting said beam, and a screen responsive to said beam, means forapplying a selected frequency of said oscillator to said deflectingmeans for moving the said beam to sweep out an ellipse on said screen atthe periodicity of said selected frequency, a pick-up device fortranslating the sound wave produced by a musical instrument being tunedinto electrical signals, an amplifier including automatic gain controlmeans for amplifying said electrical signals and maintaining themagnitude of the amplified signals at a substantially constant level inspite of variations in the magnitude of the said electrical signals, andmeans for applying the amplified electrical signals to said control gridof said cathode ray tube to intensity modulate the said electron beam asit moves to sweep out the said ellipse, whereby a non-rotating, visiblepattern is produced on said screen when an integral relation existsbetween the frequency of the said sound wave and said selected frequencysweeping out the said ellipse.

5. In musical instrument tuning apparatus, the combination of anoscillator for producing a plurality of discrete oscillations whosespacing in the frequency domain is equal to the spacing of the tones ofthe chromatic musical scale, means for adjusting said oscillator toshift said frequencies over a range as a group to reproduce thefundamental tones of a musical scale of at least two octaves, a cathoderay tube having means to generate an electron beam, and including acontrol grid for controlling the intensity of said beam, deflectingmeans for deflecting said beam, and a screen responsive to said beam,means for applying a selected frequency of said oscillator to saiddeflecting means for moving the said beam to sweep out a simple closedcurve on said screen at the periodicity of said selected frequency, apick-up device for translating the sound wave produced by a musicalinstrument being tuned into electrical signals, means for amplifyingsaid electrical signals, and means for applying the amplified electricalsignals to said control grid of said cathode ray tube to intensitymodulate the said electron beam as it moves to sweep out the said simpleclosed curve, whereby a non-rotating, visible pattern is produced onsaid screen when an integral relation exists between the frequency ofthe said sound wave and said selected frequency sweeping out the saidsimple closed curve.

6. In musical instrument tuning apparatus, the combination of anoscillator for producing twelve discrete oscillations whose spacing inthe frequency domain is equal to the spacing of the tones of thechromatic musical scale, means for adjusting said oscillator to shiftsaid frequencies over a range as a group to reproduce the fundamentaltones of a musical scale of at least two octaves and including acalibration means to indicate the accuracy of said shift in cents, acathode ray tube having means to generate an electron beam, andincluding a control grid for controlling the intensity of said beam,deflecting means for deflecting said beam, and a screen responsive tosaid beam, means for applying a selected frequency of said oscillator tosaid deflecting means for moving the said beam to sweep out an ellipseon said screen at the periodicity of said selected frequency, a pick-updevice for translating the sound wave produced by a musical instrumentbeing tuned into electrical signals, means for amplifying saidelectrical signals, and means for applying the amplified electricalsignals to said control grid of said cathode ray tube to intensitymodulate the said electron beam as it moves to sweep out the saidellipse, whereby a non-rotating, visible pattern is produced on saidscreen when an integral relation exists between the frequency of thesaid sound wave and said selected frequency sweeping out the saidellipse.

7. In musical instrument tuning apparatus, the combination of anoscillator for producing a plurality of dis,-

crete oscillations whose spacing in the frequency domain is equal to thespacing of the tones of the chromatic musical scale, means for adjustingsaid oscillator to shift said frequencies over a range as a group toreproduce the fundamental tones of a musical scale of at least twooctaves and including calibration means to indicate the accuracy of saidshift in cents, a cathode ray tube having means to generate an electronbeam, and including a control grid for controlling the intensity of saidbeam, deflecting means for deflecting said beam, and a screen responsiveto said beam, means for applying a selected frequency of said oscillatorto said deflecting means for moving the said beam to sweep out anellipse on said screen at the periodicity of said selected frequency, apick-up device for translating the sound wave produced by a musicalinstrument being tuned into electrical signals, an amplifier includingautomatic gain control means and an adjustable bandpass filter foramplifying, for maintaining the magnitude of the amplified signals at asubstantially constant level, and for allowing only those components ofthe said electrical signals that lie within the pass band of the saidadjustable bandpass filter to pass, and means for applying the amplifiedand filtered electrical signals to said control grid of said cathode raytube to intensity modulate the said electron beam as it moves to sweepout the said ellipse, whereby a nonrotating, visible pattern is producedon said screen when an integral relation exists between the filteredfrequency components of the said sound wave and said selected frequencysweeping out the said ellipse.

8. In apparatus for tuning musical instruments, an oscillator producingtwelve different frequencies whose spacing in the frequency domain isequal to the spacing of the tones of the chromatic musical scale,independent switching means for selecting one of said frequencies, saidswitching means having control buttons for the operation thereof withbuttons representing the tones of the scale being colored white andblack in accordance with the piano keyboard, the accidentals beingblack, and display means connected with said oscillator including acathode ray tube having electron beam generating and deflecting means,and a screen responsive to said beam, means connected between said beamdeflecting means and said oscillator to sweep out a simple closed curveon said screen at the periodicity of the said selected frequency, saidtube including means for the introduction of a signal for intensitymodulating the electron beam as it sweeps out the simple closed curve onthe screen, whereby a non-rotating, visible pattern is produced when anintegral relation exists between the said selected frequency and thesaid signal.

9. In musical training apparatus, the combination of an oscillator forproducing a plurality of discrete oscillations whose spacing in thefrequency domain is equal to the spacing of the tones of the chromaticmusical scale, means for adjusting said oscillator to shift saidfrequencies over a range as a group to reproduce the fundamental tonesof a musical scale of at least two octaves, a cathode ray tube havingmeans to generate an electron beam and including a control grid forcontrolling the intensity of said beam, deflecting means for deflectingsaid beam, and a screen responsive to said beam, means for applying aselected frequency of said oscillator to said deflecting means formoving the said beam to sweep out a simple closed figure on said screenat the periodicity of said selected frequency, a pick-up device fortranslating the sound wave of a musical sound into electrical signals,means for amplifying said electrical signals, means for applying theamplified electrical signals to said control grid of said cathode raytube to intensity modulate the said electron beam as it moves to sweepout the said simple closed curve, whereby a non-rotating, visiblepattern is produced on said screen when an integral relation existsbetween the frequency of the said sound wave and said selected frequencysweeping out the said simple closed curve, and an amplifier, connectedto said oscillator, the last-said amplifier driving a loudspeaker toemit an audible tone having the periodicity of said selected frequencyof the oscillator, and means to cut off the said electron beam in orderto permit blanking of said screen of said cathode ray tube.

References Cited in the file of this patent UNITED STATES PATENTS2,321,376 Finch June 8, 1943 2,340,002 McKellip J an. 25, 1944 2,442,770Kenyon June 8, 1948 2,506,971 Robinson May 9, 1950 2,537,104 Taylor Ian.9, 1951 2,577,493 Schenau Dec. 4, 1951 2,614,221 Moll Oct. 14, 19522,624,860 Baker Jan. 6, 1953 2,686,294 Hower Aug. 10, 1954 2,806,953Krauss Sept. 17, 1957 2,806,954 Tennes Sept. 17, 1957 OTHER REFERENCESOrchestral Pitch, article in The Wireless World, May 11, 1939; page 441.

